The present invention relates to a surface acoustic wave device.
Recently, surface acoustic wave devices have been used as a band pass filter, a delay line, or the like. Some examples of conventional surface acoustic wave devices will be described below with reference to FIGS. 1 through 4, of the accompanying drawings.
FIG. 1 illustrates a fundamental construction of a surface acoustic wave device serving as a band pass filter. This surface acoustic wave device comprises a substrate 1, which is made of a piezoelectric material, such as lithium niobate (LiNbO.sub.3) or lithium tantalate (LiTaO.sub.3), and is provided on the surface thereof with an input transducer 2 and an output transducer 3, each of which is composed of a pair of separated, interdigitated comb-shaped electrodes. When electrical signals from a generator 4 are applied to the electrodes of the input transducer 2, the latter emits surface acoustic waves 5, which are propagated to the output transducer 3 and then converted into other electrical signals which can be taken out from a load impedance 6. The electrodes of the input and output transducers can be designed in a variety of patterns, so as to provide various desired characteristics to the filter when electrical signals are converted into acoustic signals or vice versa. However, there is a problem that, in addition to surface acoustic waves, the input transducer also emits bulk waves which are propagated through the body of the substrate so as to reach the output transducer, adversely affecting its output characteristics.
In order to eliminate the above mentioned problem, in a conventional device, as illustrated in FIG. 2, the input transducer 2 and the output transducer 3 are arranged diagonally to each other, and a multistrip coupler 7 which includes a plurality of parallel conductors is provided between the input and output transducers. Energy of the surface acoustic waves emitted by the input transducer 2 is converted into electrical energy and then emitted again in the form of surface acoustic waves by the multistrip coupler 7, so that the path of propagation of the surface acoustic waves is changed, as indicated by a solid line. As a result, the effect of the bulk waves which are propagated, as indicated by a dotted arrow, is eliminated. In this case, however, there is a problem, as illustrated in FIG. 3, in that the surface acoustic waves are reflected by the output transducer 3, then propagated back to the input transducer 2 through the multistrip coupler 7, and are reflected again to reach the output transducer 3 through the multistrip coupler 7, thereby resulting in the occurrence of a so-called triple transit echo (hereinafter referred to as TTE), which deteriorates the characteristics of the filter.
In order to suppress such TTE, a conventional device has been proposed, as illustrated in FIG. 4. This device is provided with a reflecting transducer 8 for reflection of the surface acoustic waves. This transducer is composed of a pair of separated, interdigitated comb-shaped electrodes and is arranged opposite to the input transducer 2 with the multistrip coupler 7 therebetween. The number of conductors of the multistrip coupler 7 can be adjusted so that all of the surface acoustic waves emitted by the input transducer 2 are not caused to change their path of propagation, but a part thereof goes straight and is reflected by the reflecting transducer 8. As a result, the waves reflected by the output transducer 3 and the waves reflected by the reflective transducer 8 are cancelled by each other. The main point of this device is that the intensity of the reflected waves from the output transducer 2 and the intensity of the reflected waves from the reflecting transducer 8 are adjusted by means of a variable resistor and a variable inductor. Also the arrangement is designed so that both sets of reflected waves are in inverse phase with each other. The intensity adjustment for both sets of reflected waves of the conventional device illustrated in FIG. 4 requires a variable inductance coil 9 and a variable resistor 10, which are inserted between the electrodes of the reflective transducer 8. As a result, there is a problem in that the device is increased in size and in cost due to the variable inductance coil and the variable resistor.